Method of calibrating bimetallic elements in a thermal overload switch

ABSTRACT

This disclosure relates to a thermal bimetal operator for a multiple line control switch unit and to the method of construction. A switch housing is provided with a series of small cavities in one end wall. A bimetal strip includes an L-shaped tab which projects into the recess. A thermoset resin is disposed within the respective cavities and the bimetal strips are resiliently clamped with the outer end of the bimetal element disposed outwardly of a set position. A U-shaped trip bar is pivotally mounted within the housing with the base engaging the free ends of the bimetal strips and with an outer arm aligned with a switch blade. The switch is a snap action overcenter variety having the compensating switch arm or blade projecting laterally parallel to the trip bar and interengaged by the depending arm of the trip bar. The switch includes an adjustment screw connected to a pivot support to control the snap action position. The assembly is placed in a fixture having a support pin extending through a housing opening to support the outer arm and an adjustable tool spring loaded rod extending through an opposite housing opening into engagement with the switch blade to position the components including the bimetal strips. the fixture and assembly are placed in a curing oven which is raised to the curing temperature. Prior to the setting of the resin and after the operating temperature is established, the switch adjustment is made. The curing step is then completed.

United States Patent [72] Inventor EduardW.Isler Cleveland, Ohio [21]Appl, No. 721,755 [22] Filed Apr. 16, 1968 [45] Patented Aug. 3, 1971[73] Assignee A.O.SmithCorporation Milwaukee, Wis.

[54] METHOD OF CALIBRATING BIMETALLIC ELEMENTS IN A THERMAL OVERLOADSWITCH 3 Claims, 7 Drawing Figs.

[52] US. Cl .1 29/622, 29/407, 29/593, 29/622, 73/1, 200/166, 200/1,

[51] Int. Cl 1101b 11/00,

H0lh 1l/02,l-I01h 65/00 [50] Field of Search 29/407,

Primary Examiner-John F. Campbell Assistant Examiner-Robert W. ChurchAttorney-Andrus, Sceales, Starke and Sawall ABSTRACT: This disclosurerelates to a thermal bimetal operator for a multiple line control switchunit and to the method of construction.

A switch housing is provided with a series of small cavities in one endwall. A bimetal strip includes an L-shaped tab which projects into therecess. A thermoset resin is disposed within the respective cavities andthe bimetal strips are resiliently clamped with the outer end of thebimetal element disposed outwardly of a set position. A U-shaped tripbar is pivotally mounted within the housing with the base engaging thefree ends of the bimetal strips and with an outer arm aligned with aswitch blade. The switch is a snap action overeenter variety having thecompensating switch arm or blade projecting laterally parallel to thetrip bar and interengaged by the depending arm of the trip bar. Theswitch includes an adjustment screw connected to a pivot support tocontrol the snap action position. The assembly is placed in a fixturehaving a support pin extending through a housing opening to support theouter arm and an adjustable tool spring loaded rod extending through anopposite housing opening into engagement with the switch blade toposition the components including the bimetal strips. the fixture andassembly are placed in a curingoven which is raised to the curingtemperature. Prior to the setting of the resin and after the operatingtemperature is established, the switch adjustment is made. The curingstep is 5 6 References Cited UNITED STATES PATENTS 2,962,804 12/1960Nelsen 29/622 2,745,924 8/1958 Coates..... 200/33 3,057,047 10/1962Zimmer 29/622 3,344,250 9/1967 Duval 337/89 3,230,607 1/1966 Gelzer29/622X thencompleted.

PATENTEMUG 3197: 3,596 352 sum 1 OF 2 gal ward 15/47 METHOD OF"CALIBRATING BlIMETALLIC ELEMENTS IN A THERMAL OVERLOAD SWITCH Thisinvention relates to a thermal bimetal operator and particularly to suchan operator for a multiple line control switch unit wherein a pluralityof separate bimetal means are mounted in spaced relation and coupled tomove a common actuator.

In electrical control circuits, thermal responsive controls may beconnected in the lines to open the circuit in the event of abnormalcurrent conditions and the like. Such thermal overload controls may takea variety of different forms including bimetal elements forming acurrent-conducting contact of a switch or heated from a separate heatingelement connected in the circuit to actuate an isolated switch. Theheating of a bimetal element is directly proportional to the amplitudeor level of the current. If the current rises above a selected level,the heating effect on the bimetal element, with the well knowndifferences of thermal expansion and contraction of the individualelements, is such as to actuate the switch.

A highly satisfactory thermally activated switching system is disclosedin the copending application of Eduard W. Isler entitled ElectricalContactor Assembly, which was filed on Sept. 25, 1967, wherein separateheater means are provided in each line for actuating a common thermaloverload switch. The above application is particularly directed to athree-phase alternating current motor control having a main contactor towhich an overload switch unit is secured. A T-shaped heating unit hasone terminal connected to the overload unit and the opposite contactconnected directly to the electromagnetic main switch.

In bimetal switching elements and the like, it is often necessary tocalibrate the movement of the bimetal elements to a high degree ofaccuracy because the deflection versus temperature characteristic withinthe operating range of the bimetal is very flat. This becomesparticularly important where a plurality of separately responsivebimetal elements are provided and interconnected to operate a switchmeans through a common actuator. To provide proper actuation, each ofthe bimetal members should engage the common actuator with an equalforce for a given condition. Generally, accurate calibration has beenobtained in the past by providing calibration pins in the commonactuator and selectively fixed therein to provide the necessaryprojection toward the bimetal element to provide equal force balance.Although satisfactory switch assemblies have been provided by thismethod, the procedure is time consuming and therefore costly and cannotpractically be adapted to an automatic or semiautomatic production linemethod.

In accordance with the present invention, a switch assembly includes acommon actuator movably mounted in the path of the terminal ends of aplurality of bimetal elements. The bimetal elements are correspondinglysupported at the opposite end by a potted construction, preferably asuitable thermoset resin in such a manner as to insure equal forceengagement of each bimetal element with the common actuator for anygiven temperature condition. A completely isolated electrical switchmechanism is thereby accurately actuated in response to a giventemperature condition applied to any one or all of the bimetal elements.

In a particularly novel method of construction, the three individualbimetal elements are secured to the support with the support endsembedded within a resin in the unset or liquid state. The commonactuator is mounted to the assembly in common engagement with thebimetal elements and with the switch mechanism. The total assembly isthen mounted within a curing oven which is set at essentially theeffective operating temperature of the assembly in normal operation. Thecommon actuator and bimetal elements are then moved to a set conditionwhich just actuates the switch mechanism and constitutes the nominalsetting of the assembly. The physical interengagement of the trip leverand the switch mechanism is maintained with the three metal elements asa backup such as to maintain the desired equal force engagement: Theresin is set or cured such as to subsequently hold the mechanism in thenominal setting with equal force engagement between the elements and thecommon actuator.

The switch is preferably of a snap action overcenter variety having acompensating switch arm projecting laterally parallel to the trip barand interconnected to a depending arm of the trip bar.

In a preferred construction and novel method of assembly a switchhousing is provided with a series of small cavities to each of which oneend of a corresponding bimetal element is secured. A thermoset resin ina plastic state is disposed within the respective cavities and the oneend of each bimetal element is resiliently clamped within thecorresponding recess by a resilient clip with the outer end of thebimetal element disposed outwardly of the set position. A trip bar ispivotally mounted within the housing with a common portion engaged byeach of the bimetal elements and a spaced portion engaging a switch am.An access opening is provided in the housing to permit entrance of asuitable tool for movement of the trip bar to move the bar and thebimetal elements and thereby establish equal force engagement for anygiven set condition.

The assembly is placed in a fixture having an adjustable tool alignedwith the opening and the assembly and fixture are placed in a curingoven which is raised to the curing temperature. After the setting of theresin and after the operating temperature is established, the switchadjustment is made. The curing step is then completed. Establishing acuring temperature and affecting the calibration in the same temperaturecondition has been found to provide very accurate calibration of thebimetal elements.

The present invention thus provides a very simple, inexpensive andefficient means for accurate calibration of a plurality of bimetal orsimilar thermostatic operating elements for mov ing a common actuatormember.

The drawings furnished herewith illustrate a preferred construction ofthe present invention in which the above advantages and features areclearly disclosed as well as others which will be clear from thedescription of such embodiment.

In the drawings:

FIG. l is a plan view showing an overload relay connected to a mainelectromagnetic relay;

FIG. 2 is an exploded pictorial view of the thermal switch shown in FIG.1;

FIG. 3 is an enlarged fragmentary section through the thermal switchshown in FIGS. 1 and 2;

FIG. 4 is an enlarged fragmentary view more clearly illustrating thebimetal element mounting;

FIG. 5 is a sectional view through a switch mechanism actuated by thebimetal elements;

FIG. 6 is a view similar to FIG. 4 showing a step in the method ofassembly; and

FIG. 7 is a side elevational view partially in section to show a fixturefor proper positioning of the bimetal elements and the common actuator.

Referring to the drawings, and particularly to FIG. 1, the presentinvention is shown applied to an AC contactor 1 which is adapted to beconnected in a three-phase motor starting circuit, not shown. Anintegrated thermal overload switch unit 2 is connected to one side ofthe AC contactor 1 and is constructed to respond to load current toprevent damaging of the load or the motor components. The AC contactormay be of any desirable construction. For example, as referred to inapplicant's previously identified copending application, the maincontactor ll includes an outer insulating housing having three laterallydisplaced contact chambers 3 with line and load terminals 4 and 5,respectively, secured to the opposite ends of the chambers. The lineterminals 4 are exposed to the top and one side of the housing and areinterconnected to the load terminals 5 similarly mounted to the oppositeside of the housing by a bridging contactor assembly 6 which is slidablymounted within the housing. The assembly 6 includes bridging contacts,not shown, which are actuated by a suitable electromagnetic means or thelike. As the main contactor 1 may take any variety of forms and be ofany suitable construction, no further illustration or descriptionthereof is given.

The three corresponding terminals 4 on the line side of the contactor 1are suitably connected to the incoming power lines of a three-phasesystem whereas the load side of the contactor are interconnected throughthe thermal overload switching device 2.

Generally, the thermal overload device 2 includes a twopiece rectangularhousing of a relative deep switch portion and a shallow cover 8. Thehousing is formed of a suitable insulating material and is mounted inabutting relationship to one end face of the contactor housing of themain contactor 1. A pair of dovetailed projections and mating grooves 9interconnect the upper end of the device 2 to contactor 1. The housirgof device 2 and contactor 1 are suitably secured to a common mountingplate 10 to rigidly interlock switch device 2 to contactor 1.

The housing is divided into three heating chambers 11 aligned one eachwith the adjacent terminals of the main contactor l. The dividing wallsmay be provided with forward projecting bifurcated walls 12 mating withcorresponding vertical dividing walls 13 of the main contactor 1 toproperly locate and more firmly interconnect the housings.

The thermal overload housing is provided with three load terminals 14 onthe outer wall opposite that abutting the contactor housing with each ofthe terminals 14 aligned with one of the chambers 11 and thecorresponding terminals 5 of the contactor 1. Contact terminals arethereby provided to the opposite side of each of the heater chambers 11and support a T- shaped heating element 15 within the correspondingchamber. The heating elements 15 are constructed in accordance with theteaching of applicant's previously identified copending application andare therefore only briefly described herein.

Each T-shaped heating element 15 includes a stern including spacedconductors 16 and 17 connected at the inner end and laterally bent inopposite directions at the outer end to form contacts 18 and 19overlying terminals 5 and 14, respectively. The heater stem is locatedin the heating chamber 11 with the cross arm portion of contacts 18 and19 bridging the gap between the overload device terminal and the maincontactor terminal. The current flow in the circuit in each of the linesis through the corresponding heater element 15 to establish atemperature within the related chamber 11 in accordance with themagnitude of the current flow.

The housing 9 abutting the main contactor 1 is a relatively shallow wallmember having three shallow recesses with identical bimetal elements 21mounted in laterally spaced relation therein in accordance with thespacing of the three chambers 11. Each bimetal element 21 responds tothe tem' perature condition in the related chamber 11 to actuate acommon switch mechanism 22 housed in the lower portion of housing 3, inthe event an abnormal current overload is present for a given period.

Each of the bimetal elements 21 is similarly constructed and a singleunit is described. The element 21 includes a pair of flat metal strips23 and 24 bonded to each other and having selected different thermalcoefficients of expansion and contraction. The element 21 is securedimmediately adjacent its outer end within a cavity 25 in the housing. Inaccordance with the illustrated embodiment of the present invention, anL-shaped mounting strip 26 is secured to the element 21 and defines aninwardly projecting tab 27 which extends into cavity 25 and abuts theback wall. The cavity 25 is filled with a suitable adhesive, such as athcrmosetting resin 28 to rigidly fix the upper end of the bimetalelement 21 to the housing.

The inner ends of the three bimetal elements 21 similarly engage apivotally mounted trip member or bar 29 for actuating the switchmechanism 22. The trip bar 29 is generally a U- shaped member formed ofBakelite or any other suitable light, insulating material withupstanding arms 30 disposed one each to the opposite side of the threechambers 11. The upper ends of the arms 30 are provided with V-shapedpivot notches 31 which are supported on similar V-shaped horizontalknife edges 32 integrally formed in the housing. The trip bar 29includes the base portion 33 which spans the lower ends of the threebimetal elements 21 and is provided with bearing and wear-resistant pins34 aligned one with each of the elements 21. In the prior art, such pinsare driven into a trip bar to provide a predetermined force balancedengagement with the bimetal elements. In the present invention, theymerely provide wear surfaces.

The trip bar 29 further includes a depending arm 35 extending downwardlyfrom the base portion 29 in the opposite direction from arms 30 and intoalignment with compensating cantilevered operating arm 36 of the switchmechanism 22.

Referring particularly to FIG. 5, the illustrated switch mechanism 22 isa spring-loaded, overcenter, snap action switch having a main pivotsupport bracket 37 secured within the housing. The bracket 37 isU-shaped and opens inwardly with the web attached by a screw 38 to thefront wall of the housing and opening rearwardly within the housing. Anovercenter spring plate or blade 39 is generally U-shaped with the endsof the legs pivotally mounted in the outer side edge of the bracket 37as at 40. A coil spring 41 is secured at one end to the base of theovercenter spring blade 39 and at the opposite end to a pin 42 securedbetween the arms of the bracket 37.

The overcenter spring blade 39 has two alternate stable positions,depending upon the position of the blade with respect to the fixed pin42. The overcenter blade 39 is positioned in one position by the tripbar 29 and arm 36 and returned by a resetting lever 43, as hereinaftermore fully described.

Generally, the compensating cantilevered arm 36 extends laterally acrossthe housing from a pivot support connection to bracket 37, with theouter end aligned with the depending arm 35 of the trip bar 29. Thecantilevered arm 36 includes a generally U-shaped portion 44 telescopedinto the bracket 37 in the end opposite the overcenter blade 39. TheU-shaped portion includes apertured side arms and aligned with guideslots 45 in the U-shaped portions and includes oppositely extending tabsor arms 47 passing through the apertures and into the guide slots 45. Acalibration and adjusting screw 48 extends through the front wall of theoverload housing 7 and threads into the nut 46 with a coil spring 49encircling the screw. The tightening and loosening of the screw 48provides corresponding positioning of the pivot tabs 47 within the slot45 and therefore varies the position of the cantilevered pivot supportwith respect to the overcenter blade 39.

An operating hook or lever 50 is integrally formed with the one side ofthe U-shaped portion of cantilevered arm 36 and terminates in a bearingtip engaging the overcenter blade 39 immediately outwardly of its pivotconnection 40 to the bracket 37.

In operation, the overcenter blade has one stable position shown in FIG.5. The pivoting of the cantilevered arm 36 and the integrally connectedlever 50 cause the positive displacement of the overcenter blade 39 andthe attached spring 41 to the opposite side of center with respect topin 42.

The switch mechanism 22 further includes a blade contact 51 secured asby a rivet to the one side of the overcenter blade 39 and projectsoutwardly along the blade and beyond the outermost end of it. Contactbuttons are secured to the opposite faces of blade 51 and selectivelymake contact with normally closed contact 52 and alternatively anormally open contact 53. A backing plate or strip 54 is secured to theback side of the contact strip 51 and is shown coextensive therewith tolimit the backward deflection of the contact strip with the switchmechanism 22 in the normally closed contact position shown in HO. 5 ofthe drawings. The backing strip 54 is provided with a through contactbutton at its outer end in alignment with the contact buttons ofcontacts 51 and 53. Contact 51 is connected in circuit through thebracket 37 and a contact terminal 55 which is welded or otherwisesecured to one side of bracket 37 and extends upwardly adjacent thefront wall of the housing to the exterior, as shown in FIG. 3. The

normally closed contact 52 is a contact strip having offset and parallellegs such that it extends laterally of the housing and then forwardly tothe rear wall and along the rear wall to a terminal 56 which extendsupwardly to the exterior, as shown in FIG. 1.

The normally open contact 53 is a generally flat strip secured to theopposite front of the housing to a contact terminal 56a, as viewed inFIG. 5, and extends laterally inwardly into alignment with the contact51.

When the switch mechanism is actuated to the alternate position, thecontact 51 is connected to contact 53 and provides a current path foroperating of a trip indicator or the like, not shown. The switchmechanism 22 and particularly contact 51 is adapted to be reset by resetlever 43 which is shown as a small plastic striplike member slidablymounted adjacent the front wall of the housing adjacent contact 53 andsuitably spring loaded to continuously urge the reset lever 43outwardly. An offset cam portion 57 extends between the front wall ofthe housing and the normally open contact 53. Depressing of the resetlever 43 moves the cam portion 57 behind the normally open contact 53and pivots it outwardly around its fixed mounting to the contactterminal 56 and positively pivots the contact 51 to the reset position.

In normal operation, the switch mechanism 22 is in the normally closedposition with the cantilevered arm 36 bearing on the depending arm ofthe trip bar 29. The trip bar 29 in turn engages the bimetal elements21, which deflect from their normal position in accordance with thetemperature within the associated chamber. If for any reason, the linecurrent should rise above a normal level, the increased temperature willresult in deflection of one or more of the bimetal elements 21 beyondthe trip position causing the movement of the overcenter blade contact51 to open the normally closed contact 52 with a snap action. Thus, oncethe cantilevered arm 36 moves overeenter blade contact 51 beyond thecentered position, the contact 51 moves to the alternate position underthe force of spring 41 with a snap action movement.

In order to provide accurate calibration of the three bimetal elements21, the overload device is constructed in accordance with theparticularly novel aspect of the present invention in the followingmanner.

The shallow portion of housing 8 is formed with the clamping recesses orcavities 25. The bimetal elements 21 are secured with the L-shapedmounting tabs 26 disposed within the proper recesses which are filledwith a suitable thermosetting resin 28 in a liquid or uncured state. Thebimetal elements 211 are clamped within the recess and the uncured resinby suitable spring clips 53 with the elements angularly disposed fromthe normal operating plane, as shown in FIG. 6. The clipped assembly isdisposed on a jig 59 with the bimetal elements 21 facing upwardly andextending at an angle from the recesses 25, as shown in FIG. 7, The jig59 includes a small upwardly projecting support pin 60 which projectsinto the housing ti through a suitable opening 61 in alignment with thearm 35 of trip bar 29. The trip bar 29 is mounted on top ofthe housing53 with the three wear pins 34 engaging the corresponding ends of thebimetal elements 2i and the switch arm 35 resting on the pin 60. Theouter deeper portion 7 of switch housing is assembled with the switchmechanism 21 and then secured over the shallow housing 8 as by theattachment screws. The outer portion of housing 8 includes an adjustmentopening 62 aligned with the pin 60 of the jig, and therefore also inalignment with the outer end of the cantilevered arm 36.

A calibration adjustment rod or tool 63 is shown mounted on a post 66 asa part of jig 59 for selecting alignment with the opening or aperture62. The illustrated tool 63 is a depending rod slidably secured withinthe outer end of an overhanging arm 65 and having a knob 66 secured tothe upper end. A

' spring 67 encircles the tool rod 63 between the underside of arm 65and a small collar 68 on the intermediate portion of the rod. The tool63 is moved downwardly by the force of the spring 67 and is normallydisposed below the upper end of the pin 60.

The total assembly is now disposed within an oven 69 with the tool 63disposed to one side of the switch unit. The oven is raised to thenormal operating temperature of the bimetal elements 21. The bimetalelements 21 will then be in the normal deflected state, e.g., in anoperating state such that the switch mechanism should be tripped. Inassembly, the rod 63 is then pulled upwardly against the force of spring67, aligned with opening 62'and released to move downwardly toward pin60. The arm 35 of the trip bar 29 and the aligned arm 36 are clampeddownwardly onto the support pin 60. The arm 35 of the trip bar 29 andthe aligned arm 36 are clamped downwardly onto the support pin 60. Theswitch arm 36 is brought into physical contact with the trip lever arm35 which is backed up by the three bimetal elements 21. The elements '21are pivoted from the angular position of FIG. 6 by the force of the tool63 to establish equal force engagement between the elements 21 and thebar 29. This establishes the nominal setting of the overload relaythermal switch unit. The system is held in this position within the oven69 until the resin 28 is cured to provide a firm physical support of thetrip bar 29 and the bimetal elements in the calibrated position. Thecalibration and adjusting screw 48 is then positioned to establish thefactory calibration and a limit knob 70 may be placed on the screw topermit field adjustment.

It is found that by providing the curing and calibration under a commontemperature condition, and holding of the bimetal elements in thecalibrated position during curing, the accuracy of calibration isdirectly related to the tolerance within which the oven temperature ismaintained. The curing oven temperature can, for example, be readilymaintained within plus or minus one degree Centigrade for a curingtemperature of C.

In particular, the curing of the resin at the normal switch operatingtemperature reduces the curing time and more importantly automaticallycompensates for the bimetal parameters or manufacturing tolerances suchas thickness, deflection characteristic and width at the working oroperating temperature. The latter feature does therefore insureessentially equal force engagement of each bimetal element at thepreselected operating temperature for proper actuation of theelectrically isolated switch unit. Further, by calibrating or adjustingthe switch unit and particularly the operating arm with respect to itsengagement with the trip bar in the same temperature condition, itprovides accurate calibration of the nominal setting of the switch unit.

Iclaim:

I. The method of mounting a plurality of bimetal temperature-sensitivestrips to a support in predetermined engagement with an actuator memberand having a switch assembly with means to adjust the actuating positionwith respect to the position of the actuator member for calibrating ofthe switch assembly and securing a fixed end of each of said strips tothe support, comprising resiliently securing the fixed end of each stripto the support with each strip angularly displaced from its operatingposition and with the fixed end embedded within uncured thermosettingresin;

mounting said actuator member in simultaneous engagement with the freeends of the strips,

moving said actuator member to a selected position and therebycorrespondingly pivoting said strips about said fixed ends, and

heating said assembly to cure said thermosetting resin at a selectedoperating temperature of the switch assembly and with said actuatormember positively held in said selected position, and

positioning said switch assembly with respect to the actua tor memberduring said heating step.

2. The method of claim 1, wherein said bimetal strips are flat stripsand including forming a plurality of aligned recesses in the support,one

for each of the flat strips,

filling the recesses with a thermosetting resin,

resiliently mounting one end of a different bimetal strip in each ofsaid recesses with a resilient clamp to dispose the strips extendingangularly outwardly in an essential common plane, said clamp providingfor pivotal movement of the strips about said one end,

said actuator member including a U-shaped member formed of a rigidinsulating material and having a pair of side legs pivotally supportedat the free ends and joined by a flat base portion and a depending armand having a switch assembly including a cantilevered switch arm havinga free end aligned with said arm and a pivot end,

mounting of the actuator member with the flat base portion resting onthe outer ends of the bimetal strips to form a subassembly,

pivoting the base portion and bimetal strips about said one end andagainst the force of said clamp to a selected position within saidrecesses to establish a nominal setting of said actuator member and saidbimetal strips, and curing said resin with said assembly maintained atthe selected operating temperature and in said selected posi tion. 3. Inthe method of mounting a plurality of bimetal strip elements foractuating a common trip bar coupled to a switch unit at a selectedoperating temperature comprising forming a support plate having aplurality of aligned recesses,

filling the recesses with a thermosetting resin,

resiliently securing a corresponding fixed end of a different bimetalelement in each of said recesses with the elements extending outwardlyin a generally common plane,

mounting of the trip bar on the outer ends of the bimetal elements,

mounting said switch unit with a switch arm engaging said trip bar toform a subassembly,

heating said subassembly to a selected operating temperature,

moving said trip bar against said elements to position said trip bar andsaid bimetal strip elements and said switch arm about the fixed ends ofthe elements and locate the fixed ends within said recesses to establisha nominal setting of said trip bar,

curing said resin with said assembly maintained at the selectedoperating temperature and in said selected position, and

calibrating said switch unit by positioning of the switch arm relativeto said trip bar with said assembly maintained at a selected operatingtemperature and in said selected positron.

1. The method of mounting a plurality of bimetal temperaturesensitivestrips to a support in predetermined engagement with an actuator memberand having a switch assembly with means to adjust the actuating positionwith respect to the position of the actuator member for calibrating ofthe switch assembly and securing a fixed end of each of said strips tothe support, comprising resiliently securing the fixed end of each stripto the support with each strip angularly displaced from its operatingposition and with the fixed end embedded within uncured thermosettingresin; mounting said actuator member in simultaneous engagement with thefree ends of the strips, moving said actuator member to a selectedposition and thereby correspondingly pivoting said strips about saidfixed ends, and heating said assembly to cure said thermosetting resinat a selected operating temperature of the switch assembly and with saidactuator member positively held in said selected position, andpositioning said switch assembly with respect to the actuator memberduring said heating step.
 2. The method of claim 1, wherein said bimetalstrips are flat strips and including forming a plurality of alignedrecesses in the support, one for each of the flat strips, filling therecesses with a thermosetting resin, resiliently mounting one end of adifferent bimetal strip in each of said recesses with a resilient clampto dispose the strips extending angularly outwardly in an essentialcommon plane, said clamp providing for pivotal movement of the stripsabout said one end, said actuator member including a U-shaped memberformed of a rigid insulating material and having a pair of side legspivotally supported at the free ends and joined by a flat base portionand a depending arm and having a switch assembly including acantilevered switch arm having a free end aligned with said arm and apivot end, mounting of the actuator member with the flat base portionresting on the outer ends of the bimetal strips to form a subassembly,pivoting the base portion and bimetal strips about said one end andagaInst the force of said clamp to a selected position within saidrecesses to establish a nominal setting of said actuator member and saidbimetal strips, and curing said resin with said assembly maintained atthe selected operating temperature and in said selected position. , 3.In the method of mounting a plurality of bimetal strip elements foractuating a common trip bar coupled to a switch unit at a selectedoperating temperature comprising forming a support plate having aplurality of aligned recesses, filling the recesses with a thermosettingresin, resiliently securing a corresponding fixed end of a differentbimetal element in each of said recesses with the elements extendingoutwardly in a generally common plane, mounting of the trip bar on theouter ends of the bimetal elements, mounting said switch unit with aswitch arm engaging said trip bar to form a subassembly, heating saidsubassembly to a selected operating temperature, moving said trip baragainst said elements to position said trip bar and said bimetal stripelements and said switch arm about the fixed ends of the elements andlocate the fixed ends within said recesses to establish a nominalsetting of said trip bar, curing said resin with said assemblymaintained at the selected operating temperature and in said selectedposition, and calibrating said switch unit by positioning of the switcharm relative to said trip bar with said assembly maintained at aselected operating temperature and in said selected position.